Interpretive Summary: In agriculture, invasive plants out compete crops for soil and water resources, reduce crop quality, interfere with harvesting operations, and reduce land values. The USDA estimates the annual productivity loss of 64 crops due to invasive species at 7.4 billion. Not all introduced species however, become invasive. What are the characteristics associated with successful invasive species? Understanding these characteristics is crucial to predicting and preventing future invasions. To determine if climate change is one potential factor in the invasiveness of weedy species, six of the top 15 invasive weeds were grown at past, current and future concentrations of carbon dioxide, the principle greenhouse gas. These concentrations approximated conditions at the beginning of the 20th century, those which exist today, and those predicted for the end of the 21st century. The average stimulation of plant biomass among invasive species from current to future CO2 averaged 46 percent, with the largest response (72 percent) observed for Canada thistle. However, the growth response among these species to the recent CO2 increase during the 20th century was significantly higher, averaging 110 percent, with Canada thistle again (180 percent)showing the largest response. Overall, the stimulation of growth for tested invasive species from 285 to 382 ppm CO2 was about 3x greater than for any species examined previously, suggesting the possibility that the recent increase in CO2 during the 20th century may have been a factor in the invasiveness of these species.

Technical Abstract:
The response of plant species to future atmospheric carbon dioxide (CO2) concentration has been determined for hundreds of crop and tree species. However, no data is currently available regarding the response of invasive weedy species to past or future atmospheric CO2. In the current study the growth of six species which are widely recognized as among the most invasive weeds in the continental United States, Canada thistle (Cirsium arvense), field bindweed (Convolvulus arvensis), leafy spurge (Euphorbia esula), perennial sowthistle (Sonchus arvensis), spotted knapweed (Centaurea maculosa), and yellow star thistle (Centaurea solstitialis) were grown from sowing at either 284, 380 or 719 parts per million (ppm) CO2 until the onset of sexual reproduction. The CO2 concentrations corresponded roughly to the CO2 levels which existed at the beginning of the 20th century, the current level of atmospheric CO2, and the CO2 concentration projected for the end of the 21st century, respectively. The average stimulation of plant biomass among invasive species from current to future CO2 averaged 46 percent, with the largest response (72 percent) observed for Canada thistle. However, the growth response among these species to the recent CO2 increase during the 20th century was significantly higher, averaging 110 percent, with Canada thistle again (180 percent) showing the largest response. Overall, the stimulation of growth for these species from 285 to 382 ppm CO2 was about 3x greater than for any species examined previously, suggesting the possibility that the recent increase in CO2 observed during the 20th century may have been a factor in the invasiveness of these species.